This invention relates to a method for recovering polymer resins from solution. More particularly, it relates to a continuous process for isolating a polymer resin from an organic solvent solution feed by admixing the solution with an aqueous solution and volatilizing the organic solvents to provide solid polymer granules of a high bulk density.
There are several methods which have been suggested for separating thermoplastic resins from organic solutions. The majority of these processes are energy intensive, requiring large quantities of steam to accomplish solvent removal. Those separation processes which are energy conservative either provide solid product of a low bulk density, provide particulate product with undesirable impurities or require complex operating equipment.
The processes described in U.S. Pat. Nos. 4,408,040 and 4,423,207 provide methods for separating polymers from solution wherein said solution is atomized with steam within an agitated vessel of hot water. The solvent is flashed off within the hot water leaving a slurry of solid polymer. These processes provide an energy efficient means for separating the polymer from solution; however, these processes require unique equipment, such as a spray nozzle designed to disperse the polymer. In addition, a polymer particulate of a higher bulk density, a narrower size distribution and lower concentration of retained volatiles is desired.
The process described by Kohyama et al. in U.S. Pat. Nos. 4,452,976 obtains polymer products of high density by feeding a polymer solution into water in conjunction with a recycled product slurry which has been treated by a wet pulverization process. The polymer solution is then devolatilized within the water/product slurry. This process is very energy intensive in that it requires a high recycle rate to prevent clogging of the apparatus. In addition, complex equipment is required (wet pulverizer) to perform the process.
Methods which utilize simplified equipment to obtain polymer solids of a higher bulk density are described in copending application Ser. No. 751,777. In this process, separation of polymer resin is achieved by admixing an organic solution of the polymer with an aqueous slurry of polymer particles and volatilizing the organic solvents in solution. This process is efficient and provides good granular product; however, there is room for improvement. To operate continuously, a continuous source of an aqueous polymer slurry is required to avoid the formation of large polymer granules. These polymer particles provide agglomeration sites which are the starting point for granule formation. Without the addition of these polymer particles, polymer agglomeration sites will form initially and continue to grow within the admixture without providing new agglomeration sites. As granules are recovered from such a process, the unrecovered granules grow forming large, undesirable granules. A separate apparatus is needed to provide a continuous source of this slurry.
In an embodiment of the invention described in Ser. No. 751,777 a separate source for the aqueous slurry of polymer particles is not required. However, this embodiment operates semi-continuously to avoid the formation of large granules. Upon formation of particles of a desired size, the process is stopped.
A continuous process which does not require a source of an aqueous slurry of polymer particles to provide high bulk density polymer granules with a narrow size distribution is desired.